High-performance transparent all-carbon photodetectors based on the semiconducting single-walled carbon nanotube/fullerene heterojunctions

ZHANG Luo-xi，YIN Huan，CHEN Yue，ZHU Ming-kui，SU Yan-jie

（Key Laboratory of Film and Microfabrication (Ministry of Education), School of Electronics, Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China）

* Corresponding author，E-mail: yanjiesu@sjtu.edu.cn

Abstract: Taking advantage of the high absorption coefficient, excellent photoelectric properties, and high carrier mobility of Single-Walled Carbon NanoTubes (SWCNTs), high-performance, transparent, all-carbon Field-Effect Transistor (FET) photodetector has been constructed with a high transmittance more than 80% in the visible light band, in which semiconducting SWCNT (sc-SWCNT)/fullerene (C60) heterojunctions as the channel materials, patterned metallic SWCNT film as source/drain electrodes, graphene oxide (GO) as the dielectric layer, and Indium Tin Oxide (ITO) as a transparent gate electrode.The electrical test results show that the photodetector exhibits a strong gate-tunable characteristics, and achieves a broadband spectral response from 405 to 1 064 nm in the visible-near infrared spectral region.Under 940 nm illumination with a light density of 5 mW/cm2, the maximum photoelectric responsivity of 18.55 A/W and a specific detectivity of 5.35×1011 Jones can be achieved.

Key words: single-walled carbon nanotubes; Fullerene; all-carbon heterojunctions; high transparency; fieldeffect transistor photodetector

1 Introduction

Due to the rapid development of semiconductor technology and information science, the research on photosensitive devices has received extensive attention.Photosensitive devices play a crucial core role in modern optical detection, optical communication, optical information processing, and optical control technologies in industrial technology, national defense, military, and civilian fields.As the scale and diversity of applications are increasing,the demand for light detection devices with higher speed, high conversion efficiency or wide wavelength range, flexibility, and transparency is becoming more prominent.Moreover, there are increasing requirements for the operational performance of photodetectors, such as high sensitivity and responsivity as well as fast response speed, low noise and low power consumption of the devices in the operating wavelength band[1].Kobe University[2]has developed an infrared sensor with high responsivity at RT, and the central part of the device is an Al0.3Ga0.7As/GaAs heterostructure.The maximum photoelectric responsivity of 0.8 A/W and a specific detectivity of 1.8×1010Jones are achieved at about 6.6 μm at a bias of 1V.Although conventional silicon-based photodetectors have the advantages of mature preparation process and low cost, the wide band gap of silicon materials (≈1.12 eV) limits the range in working wavelength[3].In addition, the energy band structure of the indirect band gap of silicon material makes it impossible to achieve high-efficiency photoelectric conversion, especially in the field of transparent optoelectronics.In this field, the light absorption capacity of materials such as silicon, germanium, indium gallium arsenic and other materials with high transparency is drastically reduced, making it difficult to achieve the perfect integration of high transparency and high optoelectronic performance.At present, most photoactive materials used in photodetectors are inorganic, and the manufacturing process of these materials requires high temperature and high energy consumption, and the growth process needs to use many complex methods.These methods are complex,sensitive to process fluctuations, and have high technical requirements.In addition, the processes and photoactive materials themselves typically contain harmful elements such as lead, mercury, cadmium and arsenic.Therefore, the development of NIR photodetectors based on new materials has gradually become a research focus in recent years.

Since the 1960s, the emerging field of organic electronics has made tremendous progress in catching up with inorganic semiconductor technology and now offers alternatives for many optoelectronic applications.The development of inorganic materials is currently dominated by inorganic semiconductors or metals, such as transparent electrodes, thin-film transistors, solar cells, and photodetectors.Among them, low-dimensional nanoscale materials have attracted much attention for their potential applications in new printable, highly integrated flexible and self-powered photochemical UV-NIR broad-spectrum photodetectors.

In recent years, allotrope structures of carbon such as fullerenes (C60), carbon nanotubes, and graphene have attracted a great deal of research interest and experimental applications due to their superior chemical, physical, mechanical, and electronic properties.Depending on the chemical properties,some of these carbon materials are metallic and some are semiconducting and can form insulating oxides.Therefore, the use of these materials in combination to fabricate new optoelectronic devices composed entirely of carbon-based materials offers some attractive possibilities for the development of next-generation electronic devices.Carbon-based materials are very abundant on Earth[3-4]and can be dispersed and deposited using solution processes, so they can be used directly in well-developed tools and processes[5], and these advantages lay the foundation for the development of carbon nanomaterials for research and applications.Due to their excellent electrical conductivity, high transparency and high robustness, carbon nanomaterials have received high attention, especially Single-Walled Carbon Nano Tubes (SWCNTs).As a typical quasi one-dimensional nanomaterial, SWCNTs have special electrical and optical properties[6-7]and have been extensively investigated in various application fields, such as transistors and solar cells[8-9].According to their diameters and chirality, SWCNTs exhibit semiconducting or metallic characteristics[10].The band gaps of semiconducting SWCNTs (sc-SWCNTs) with different diameters vary from 0.5 to 1.2 eV.Due to their ultra-high carrier mobility (105cm2/Vs), high absorption coefficients (104～105/cm), and long exciton diffusion length, sc-SWCNTs are commonly used as active materials for high-performance carbon-based photodetectors[11-12].In addition, the electron transition of sc-SWCNTs is sensitive to polarized light due to their specific angular momentum in its subband gap, thus further expanding the detection applications of sc-SWCNTs-based photodetectors[13-14].Due to the above unique properties, sc-SWCNTs have become an ideal material for light energy collection in broadband light detection.

There have been numerous reports on the research and applications of various photosensitive devices based on sc-SWCNTs.Researchers from Peking University[15]have developed an asymmetric structure based SWCNT photovoltaic type IR detector with a responsivity of 9.87×10-5A /W and a detectivity of 107Jones.This type of IR detector has the advantages of simple process and no cooling IR detection at RT.L Peng's team[16]also reported a high-performance photodiode based on carbon nanotubes treated by a dopant-free technique solution,which can operate at RT and zero bias.The broadband response range of the detector is 785 ～2 100 nm, and the detectivity exceeds 1011Jones.However, the photogenerated electron/hole pairs in sc-SWCNTs usually remain in the exciton state, and the separation of excitons usually requires a strong electric field or an internal electric field to generate photocurrents in the external circuit.Therefore, the dissociation and transfer of excitons need to be enhanced by combining with other materials such as bulk semiconductors, nanomaterials, and polymers to form heterojunctions[17-19].Due to its spherical structure, C60has a high electron affinity and requires less recombination energy during electron transfer.Therefore, C60tends to accelerate forward electron transfer and slow down reverse electron transfer, resulting in long-lived charge-separated states[20-22].In various optoelectronic applications,C60is commonly used as an efficient trapping material for photogenerated electrons, which generates higher photocurrents by trapping light-generated carriers and enabling longer carrier recombination lifetimes[23-24].In addition, the all-carbon heterojunction constructed from sc-SWCNTs and C60can also avoid interfacial atomic layer diffusion to a certain extent, which is more favorable for the dissociation of photogenerated electron-hole pairs[18].A novel photodetector based on graphene nanoribbons-C60heterostructures is prepared by Prof.Wang's group at Nanyang Technological University, Singapore.It can achieve a high photoresponsivity of 0.4 A/W under mid-infrared laser irradiation at room temperature, which enhanced the photoresponsivity of the transistor by about an order of magnitude over that of pure graphene[21].This high performance is achieved by the high electron capture efficiency of the C60film deposited on the graphene nanoribbon.Such carbon material heterojunction photodetectors pave the way for the realization of flexible and broadband photodetectors for various applications such as imaging, remote sensing, and infrared camera sensors.On the other hand, field-effect transistors usually use gold as the electrode material,however, the use of gold will decrease optical transparency, while the use of metal-free devices such as SWCNT can not only achieve optical transparency and mechanical robustness, carbon-based conductive material also have advantages over other metal contacts in electrical contact carbon nanostructures.

Therefore, this paper constructs a transparent,all-carbon field-effect transistor-type photodetector based on sc-SWCNT/ C60heterojunctions, metallic SWCNTs as source-drain electrodes, graphene oxide (GO) as the dielectric layer, and indium tin oxide (ITO) as the transparent gate.The high transmittance of the device was demonstrated by characterizing the light transmission of the sample by UVVis-NIR spectrophotometer.The modified sc-SWCNT material is characterized and analyzed by scanning electron microscopy and Raman spectroscopy for microscopic morphology and charge transfer level, and the results show that C60played a p-type doping role for sc-SWCNT.The electrical tests demonstrate that the device has a more sensitive photoelectric response to visible-near-infrared light in the 405～1 064 nm band, expanding the application of the photodetector in next-generation transparent technologies such as smart windows and artificial intelligence glasses.

2 Experiment

2.1 Preparation of sc-SWCNT/ C60 all-carbon devices

First, the (6,5) SWCNT dispersion was prepared.0.5 mg of (6,5) SWCNT powder (Sigma-Aldrich) was weighed and dispersed into 10 mL of aqueous sodium dodecyl sulfate (SDS) solution(0.01 g/mL).After 2 h of ultrasonic treatment in an ice bath, the inadequately dispersed (6,5) SWCNT powder was removed by centrifugation at 14 000 rpm for 30 min, and the supernatant after centrifugation was diluted 5 times to obtain a homogeneous (6,5)SWCNT dispersion.Then, GO films were prepared by vacuum extraction and filtration method and used as the dielectric layer of the transistors.After diluting the GO aqueous solution and sonicating at low temperature for 1 h, 10 mL of the dispersion was gradually added to a vacuum filtration device containing a 0.22 μm cellulose membrane and filtered to form a homogeneous GO film.After filtering the aqueous solution, excess deionized water was added to clean the excess SDS in the film 3 times to reduce its effect on the film performance.Finally, the GO films on the cellulose membranes were dried in a vacuum oven at 40 °C for 2 h.The aqueous graphene oxide solution was replaced with a homogeneous dispersion of metallic SWCNT in deionized water (0.05 mg/mL).Vacuum filtration was performed in the same way as described above to obtain uniform and dense conductive sc-SWCNT films as a backup material for the source-drain electrode.

ITO conductive glass with a thickness of 135 nm was used as the substrate and the gate electrode.First, the ITO conductive glass was cleaned with deionized water, acetone, isopropanol and ethanol in order to remove oil from the substrate surface.Then, the GO film on the prepared cellulose film was transferred to the ITO substrate, and the GO film was laminated and spread on the conductive glass surface using ethanol and water, and then the cellulose film was dissolved in acetone by soaking in acetone, and washed repeatedly with acetone to prevent the residual cellulose film on the substrate from affecting the device performance.Then,the above prepared sc-SWCNT dispersion was deposited on the GO surface by spin coating method at 2000 rpm to form a uniform film, and 2 mg of C60was weighed by vacuum thermal evaporation method to make uniform vaporization onto the surface of the carbon tube film.The heterojunction of (6, 5)SWCNT/C60on the substrate was constructed by annealing at 60 °C for 1 h under vacuum.Finally, the metallic SWCNT film obtained by suction filtration was transferred to the sc-SWCNT/C60heterojunction by repeating the above steps to form a sourcedrain pattern.The channel width between the source-drain electrodes is 200 μm and the length is 400 μm.

2.2 Testing and characterization

In this paper, the surface morphology of sc-SWCNT films/C60heterojunctions as well as metallic SWCNT was characterized using a scanning electron microscope (SEM, Zeiss Ultra Plus, Germany).The transmission spectra were obtained by characterizing the transmittance of the samples to light using a Lambda 950 model UV-Vis-NIR spectrophotometer (USA).The Raman peak shifts of the heterojunction were analyzed by Raman spectroscopy statistics at an excitation wavelength of 514 nm.The optoelectronic properties of the devices were evaluated at room temperature using a semiconductor parameter analyzer, and the currentvoltage (I-V) curves and current-time (I-T) curves of the devices were measured under the irradiation of various monochromatic laser diodes with adjustable power as the signal source of the optical pulses (laser wavelengths including 405, 532, 650,780, 860, 940 and 1064nm).

3 Results and discussion

The structure diagram of the constructed fieldeffect transistor is shown in Figure 1(a), where ITO and GO form the gate and dielectric layer, sc-SWCNT/C60heterojunction serves as the conductive channel, and metallic SWCNT forms the sourcedrain electrode.Figure 1(b) shows the optical transmission spectrum within the visible light range of the channel (sc-SWCNT/C60film) region.The inset shows the physical image of the prepared field-effect transistor device.It is obvious that almost all carbon material films deposited on ITO are completely transparent, and their transmittance is only 10% lower than that of the substrate.When 80% of the substrate is covered with sc-SWCNT film (forming channels and electrodes), the optical transmittance of the device remains above 80%, indicating that the design of the device does not affect the light absorption of the channel layer itself.Figure 2(a)shows the carbon nanotubes deposited on GO by spin coating, sc-SWCNT is evenly distributed throughout the entire region, and no obvious surface dispersants or other polymers are visible in the image,demonstrating the high dispersibility of the nanotubes.The carbon tube solution dispersed and centrifuged by ultrasound ensures the uniformity and density of the sc-SWCNTs film.Figure 2(b) shows the SEM image of the sc-SWCNT/C60composite film, from which it can be seen that C60is uniformly distributed in the sc-SWCNT film.Figure 2(c) shows the SEM image of the metallic SWCNT as the source-drain electrode.It can be seen that a uniform and dense metallic SWCNT film was formed by suction filtration, which ensures good conductivity of the source-drain electrode.

Raman spectroscopy is the most used tool for studying carbon nanotubes.Raman spectroscopy can be used to analyze the doping effect of other materials on carbon nanotubes[25].The change in doping level can be analyzed by observing the change in G and 2D peaks in Raman images.Figs.3(a) and 3(b) (color online) show the Raman spectra of pristine and C60-doped sc-SWCNT films in G and 2D modes.For sc-SWCNT, the D-peak in the 1 300～1 400 cm-1region originates from the sp3defect in the carbon atom[26].The strong tangential mode (G-peak) near 1 580 cm-1arises from the inplane vibrations of the sp2hybridized C=C bond in sc-SWCNT.sc-SWCNTs G-peaks consist of both G+(LO) and G-(TO) modes[27], and the G+peaks represented by the analyzed LO phonons as well as the analog peaks of the 2D peaks are shown in light gray in the figure.After the addition of C60to the sc-SWCNTs, a tendency to broaden and shift the two main peaks, G+and 2D, to higher energies is observed, and this change can be attributed to the doping effect of C60on the sc-SWCNTs.It can be observed that the half-peak width of the G+peak of sc-SWCNT broadens from the original 20 cm-1to 24 cm-1after the addition of C60.In addition, the peak position shifts to the right by 2.5 cm-1, and the Raman spectrum of the 2D peak also produces a broadening of the half-peak width (about 4cm-1) and a slight blue shift (from 2 634 cm-1for pure sc-SWCNT to 2 636 cm-1after C60doping cm-1) changes.These changes may be an indication of the p-type doping caused by C60resulting in the transfer of electrons from the valence band of sc-SWCNT to C60[28].This suggests that there is a charge transfer between sc-SWCNT and the surrounding C60, which causes p-type doping of (6,5)SWCNT.

The Raman analysis of sc-SWCNT/ C60films described above has initially elucidated the charge transfer mechanism of the heterojunction channel of the prepared field-effect transistor and the operating mechanism.First, excitons are generated when the sc-SWCNTs are irradiated under light.Since the Lowest Unoccupied Molecular Orbit (LUMO) of C60is lower than the conduction band of the sc-SWCNT, the photoexcited electrons of C60cannot be transferred to the conduction band of the sc-SWCNT[29].In addition, since the offset energy difference between LUMOC60and sc-SWCNT conduction band is greater than the exciton binding energy generated, electrons in the valence band of sc-SWCNT can be transferred into the Highest Occupied Molecular Orbital (HOMO) of C60[30], producing charge separation, i.e., the transfer of photo-generated electrons from sc-SWCNT to the surrounding C60, while the hole in sc-SWCNT density increases.As a result, the photogenerated excitons in the sc-SWCNT are effectively dissociated due to the structure of the designed heterojunction, reducing the recombination rate of photogenerated excitons[31-32].This effective exciton collection allows the device to exhibit excellent optoelectronic properties.

The electrical performance of this heterojunction device is shown in Figure 4.With a constant source-drain voltage (Vds) (Vds= 0.1V), the sourcedrain current (Ids) can be changed by adjusting the carrier concentration in the channel by changing the gate voltage (Vgs), resulting in a transfer characteristic curve, as shown in Fig.4(a).As can be seen from the figure, the turning point of the characteristic curve is aboutVgs=1 V, indicating that the channel material sc-SWCNTs used during this period are p-type doped.When the applied gate pressure is less than the turning point, i.e.,Vgsis on the left, more and more holes are induced in the channel as the absolute value of the gate pressure increases.As a majority carrier, the source-drain current of the device increases rapidly.On the contrary, the closer the gate voltage is to the turning point, the lower the concentration of holes in the channel, and the source-drain current decreases until the device shuts down.However, because the channel material is pdoped, the device cannot be completely shut down and there will still be an off-current of about 0.5 nA.When the gate voltage increases again above the turning point, electrons are induced in the channel,and the electron concentration increases with the further increase of the gate pressure, gradually replacing the hole to become the majority carrier, and gradually increasing the source-drain current in the opposite direction.The transconductance of the device is calculated to be 2.268×10-8S, with a switching ratio of up to 500.The output characteristic curve of the device is obtained by linearly scanning the source-drain voltageVdsfrom -1 V to 1 V while remaining the gate voltageVgsconstant.A cluster of output characteristic curves is obtained by testing the output characteristics for a range ofVgsvalues, as shown in Figure 4(b) (color online).It can be seen that the source-drain current shows a clear separation at different gate voltages, indicating that the device has a strong gate control capability.Besides, the device is bridged between source and drain electrodes by sc-SWCNT/C60composite film,and the special electrical and optical properties of the all-carbon heterojunction provide a material basis for high-performance wide spectrum photodetection.Therefore, in this paper, the optoelectronic properties of the devices were investigated in room temperature air, as shown in Figs.5(a) and 5(b)(color online).In addition, the I-V curves of the heterojunction devices under laser irradiation at different wavelengths and the I-T curves under unbiased voltage are shown, respectively.In order to test the photoelectric performance of the device, the sourcedrain current under a linear sweep of the bias voltageVdsfrom -1 V to 1 V was tested using a semiconductor parametric analyzer at 405, 532, 650,780, 860, 940, and 1 064 nm wavelengths (all laser powers were 5 mW/cm2), respectively.We observed that at a bias voltage of 1 V, an induced photocurrent of about 0.7 μA can be generated.When the laser is repeatedly turned on and off, the photocurrent increases and decreases sharply respectively,that is, the photodetector can show good repeated switching performance between switching states, indicating that the device has excellent reliability and cycle stability.In addition, the device exhibits significant optical response in the laser wavelength range of 405-1 064 nm, proving its wide spectral photoelectric response from visible light to near-infrared,and demonstrating certain advantages in the near-infrared band.In order to further evaluate the photoelectric performance of the heterojunction device,the responsivity and detectivity under different wavelength laser irradiation were calculated based on the measured photocurrent.The responsivity is defined as the photocurrent generated per unit area per unit illumination laser power density.The calculation formula is, whereIpis the induced photocurrent,Idis the dark current without laser irradiation,Pis the optical power density, andAis the area of the effective channel area of the device.The detectivity is generally used as an indicator of the detector's performance to detect the minimum optical signal, which can be used to evaluate the sensitivity of the device.The corresponding calculation formula is, whereqis the elementary charge,Ris light responsiveness.It is calculated that the device can achieve the responsivity of 18.65 A/W and the detectivity of 5.35×1011Jones under 940 nm laser irradiation at a power density of 5 mW/cm2.The good photoelectric response performance is due to the ultra-high carrier mobility of sc-SWCNT, excellent optical absorption coefficient,long exciton diffusion length, and effective dissociation of excitons by C60and transfer collection.

4 Conclusion

Taking advantage of the unique electronic properties and high carrier mobility of sc-SWCNT,a transparent, all-carbon field-effect transistor-type photodetector is designed and prepared in this paper.The components of this field-effect transistor (active channel, source-drain electrode, and dielectric layer) are all composed of carbon-based materials,and the transmittance of visible wavelengths are all higher than 80%.In addition to the large absorption coefficients and high conduction paths provided by

——中文對(duì)照版——

1 引 言

由于半導(dǎo)體技術(shù)和信息科學(xué)的快速發(fā)展，光敏器件的研究得到廣泛關(guān)注和重視。光敏器件在現(xiàn)代光探測(cè)、光通信、光信息處理和光控制等工業(yè)技術(shù)、國(guó)防軍事和民用光電技術(shù)領(lǐng)域起著關(guān)鍵作用。隨著應(yīng)用領(lǐng)域的規(guī)模和多樣性的增長(zhǎng)，對(duì)具有更高速度、更快轉(zhuǎn)化效率或更寬波長(zhǎng)范圍以及高靈活性、高透明度的光檢測(cè)器件的需求正變得越來(lái)越強(qiáng)烈。此外，對(duì)光電探測(cè)器的工作性能也提出越來(lái)越高的要求，如在工作波段下器件要有高靈敏度、高響應(yīng)度且要具有快速響應(yīng)和低噪聲、低功耗等性能[1]。神戶大學(xué)的學(xué)者[2]研發(fā)了在室溫下具有高響應(yīng)性的紅外傳感器，裝置的中心部分是一個(gè)Al0.3Ga0.7As/GaAs 異質(zhì)結(jié)構(gòu)，在1 V 偏置下，在約 6.6 μm 處的最大值為0.8 A/W，比探測(cè)度為1.8×1010Jones。傳統(tǒng)硅基光電探測(cè)器雖然具有制備工藝成熟、成本低等優(yōu)勢(shì)，然而硅材料較寬的帶隙（≈1.12 eV）限制了其光電工作的波長(zhǎng)范圍[3]，另外，硅材料間接帶隙的能帶結(jié)構(gòu)使得the sc-SWCNTs, the proper energy band alignment between the sc-SWCNT and C60interface and the separation of the photogenerated electron-hole pairs also lead to a significant enhancement of the photoelectric response performance of the device.Through the electrical test of the device, it has been proven that the device has good control ability over gate voltage.The responsivity and detectivity at 940 nm can reach 18.65 A/W and 5.35 × 1011Jones, respectively, and a broad spectral response from 405 to 1 064 nm can be achieved, demonstrating the great advantage of sc-SWCNT/ C60film heterojunctions for photodetectors.The results presented in this paper provide a new idea for the design and preparation of high-performance all-carbon-based optoelectronic devices, and provide a strong support for the development of next-generation of all-carbon transparent optoelectronic technology.它無(wú)法實(shí)現(xiàn)高效率的光電轉(zhuǎn)換。特別是在透明光電子領(lǐng)域，透明度較高的硅、鍺、銦鎵砷等材料的吸光能力大幅度下降，難以實(shí)現(xiàn)高透明度和高光電性能的完美融合。目前，大多數(shù)光電探測(cè)器所使用的光活性材料都是無(wú)機(jī)的，這些材料的制造需要經(jīng)過(guò)幾百攝氏度的高溫、高能耗過(guò)程，生長(zhǎng)過(guò)程中需要很多復(fù)雜方法。這些方法工藝復(fù)雜，對(duì)工藝的波動(dòng)敏感，并且具有很高的技術(shù)要求。此外，工藝過(guò)程和光活性材料本身通常含有鉛、汞、鎘和砷等有害元素。因此，開(kāi)發(fā)基于新材料的近紅外光電探測(cè)器逐漸成為近年來(lái)的研究重點(diǎn)。

自 19 世紀(jì)60 年代以來(lái)，新興的有機(jī)電子領(lǐng)域取得了巨大的發(fā)展，不斷追趕無(wú)機(jī)半導(dǎo)體技術(shù)，如今已經(jīng)為許多光電應(yīng)用提供了替代方案。無(wú)機(jī)材料目前主要以無(wú)機(jī)半導(dǎo)體或金屬為主，如構(gòu)成透明電極、薄膜晶體管、太陽(yáng)能電池以及光電探測(cè)器等。其中，低維納米級(jí)材料因其在新型可印刷、高集成度柔性和自供電光電化學(xué)紫外-近紅外寬光譜光電探測(cè)器中的潛在應(yīng)用而受到廣泛關(guān)注。

近年來(lái)，富勒烯（C60）、碳納米管、石墨烯等碳的同素異形體結(jié)構(gòu)因其優(yōu)越的化學(xué)、物理、機(jī)械和電子性能吸引了大量研究人員的興趣，進(jìn)行了大量實(shí)驗(yàn)應(yīng)用。由于化學(xué)性質(zhì)的不同，這些碳材料有些是金屬性的，有一些是半導(dǎo)體性的，并且可以組成絕緣的氧化物。因此，通過(guò)這些材料組合使用來(lái)制造完全由碳基材料組成的新型光電器件為下一代電子器件的發(fā)展提供了一些有吸引力的可能性。碳基材料在地球上的含量非常豐富[3-4]，并且可以使用溶液工藝進(jìn)行分散和沉積，因此它們可以直接用于已成熟的工具和工藝中[5]。這些優(yōu)勢(shì)為碳納米材料的研究和應(yīng)用奠定了基礎(chǔ)。碳納米材料由于具有優(yōu)異的導(dǎo)電性、高透明度和高魯棒性而受到高度關(guān)注。單壁碳納米管（SWCNT）作為典型的準(zhǔn)一維納米材料，具有特殊的電學(xué)和光學(xué)特性[6-7]，已經(jīng)在多種應(yīng)用領(lǐng)域中得到了廣泛的研究，如晶體管、太陽(yáng)能電池等[8-9]。根據(jù)其直徑和手性的不同，SWCNT 表現(xiàn)出半導(dǎo)體或金屬的特征[10]。不同直徑的半導(dǎo)體性SWCNTs（sc-SWCNTs）的帶隙不同，從0.5 到1.2 eV 不等。由于sc-SWCNTs 具有超高的載流子遷移率（105cm2/Vs）、高吸收系數(shù)（104～105/cm）和較長(zhǎng)的激子擴(kuò)散長(zhǎng)度，使得sc-SWCNTs 被普遍用于高性能碳基光電探測(cè)器的活性材料[11-12]。此外，由于sc-SWCNTs 子帶隙中的特定角動(dòng)量，其電子躍遷對(duì)偏振光敏感，這進(jìn)一步提升了基于sc-SWCNTs 的光電探測(cè)器的檢測(cè)應(yīng)用[13-14]。由于上述獨(dú)特的特性，sc-SWCNTs 已經(jīng)成為寬帶光探測(cè)中用于光能收集的理想材料。

基于sc-SWCNTs 的各種光敏器件的研究和應(yīng)用已有大量的報(bào)道。北京大學(xué)[15]開(kāi)發(fā)了一種基于非對(duì)稱結(jié)構(gòu)的SWCNT 光伏型紅外探測(cè)器，該探測(cè)器的響應(yīng)率為9.87×10-5A/W，探測(cè)率為107Jones。這種類型的紅外探測(cè)器具有工藝簡(jiǎn)單和室溫下的無(wú)需冷卻紅外探測(cè)的優(yōu)勢(shì)。彭練矛團(tuán)隊(duì)[16]還報(bào)道了一種通過(guò)無(wú)摻雜技術(shù)溶液處理碳納米管基的高性能光電二極管，該二極管可以在室溫和零偏壓條件下工作。探測(cè)器的寬帶響應(yīng)范圍為785 ～ 2 100 nm，探測(cè)率超過(guò)1011Jones。然而，sc-SWCNTs 中的光生電子/空穴對(duì)通常保持在激子狀態(tài)，激子的分離通常需要強(qiáng)電場(chǎng)或內(nèi)部電場(chǎng)，才能在外部電路中產(chǎn)生光電流。因此，需要通過(guò)與其他材料（如體半導(dǎo)體、納米材料和聚合物）結(jié)合形成異質(zhì)結(jié)來(lái)增強(qiáng)激子的解離和轉(zhuǎn)移[17-19]。由于為球形結(jié)構(gòu)，C60在電子轉(zhuǎn)移過(guò)程中具有較高的電子親和度，需要較少的重組能量。因此，C60傾向于加速正向電子轉(zhuǎn)移，減緩反向電子轉(zhuǎn)移，從而形成長(zhǎng)壽命的電荷分離態(tài)[20-22]。在各種光電子應(yīng)用中，C60被普遍用作光致電子的高效捕獲材料，該材料通過(guò)捕獲光產(chǎn)生的載流子，使載流子重組壽命延長(zhǎng)，從而產(chǎn)生更高的光電流[23-24]。另外，由sc-SWCNTs 和C60構(gòu)筑的全碳異質(zhì)結(jié)還可以一定程度上避免界面間的原子層擴(kuò)散，更有利于光致電子空穴對(duì)的解離[18]。新加坡南洋理工大學(xué)王岐捷教授團(tuán)隊(duì)制備了一種基于石墨烯納米帶-C60雜化納米結(jié)構(gòu)的新型光電探測(cè)器，在室溫下，在中紅外激光照射下能夠?qū)崿F(xiàn)0.4 A/W 的高光響應(yīng)率，比純石墨烯的晶體管光響應(yīng)增強(qiáng)了約一個(gè)數(shù)量級(jí)[21]。這種高性能就是通過(guò)沉積在石墨烯納米帶上的C60薄膜的高電子捕獲效率實(shí)現(xiàn)的。這種碳材料異質(zhì)結(jié)光電探測(cè)器為實(shí)現(xiàn)柔性和寬帶光電探測(cè)器的各種應(yīng)用鋪平了道路，如成像，遙感和紅外相機(jī)傳感器。另一方面，場(chǎng)效應(yīng)晶體管通常使用金作為電極材料，但是金的使用降低了光學(xué)透明度，采用SWCNT 等無(wú)金屬器件不僅可以實(shí)現(xiàn)光學(xué)透明度和機(jī)械魯棒性，碳基導(dǎo)電材料在電接觸碳納米結(jié)構(gòu)方面也比其他金屬觸點(diǎn)更有優(yōu)勢(shì)。

因此，本論文利用碳納米材料獨(dú)特的性能，基于sc-SWCNT/C60異質(zhì)結(jié)，以金屬性SWCNT 作為源漏電極，以氧化石墨烯（GO）作為介電層，氧化銦錫（ITO）作為透明柵極，構(gòu)筑了一種透明、全碳場(chǎng)效應(yīng)晶體管型光電探測(cè)器。通過(guò)紫外-可見(jiàn)-近紅外分光光度計(jì)表征了樣品對(duì)光的透過(guò)率，證明了器件具有很高的透光率。通過(guò)掃描電子顯微鏡及Raman 光譜儀對(duì)修飾后的sc-SWCNT 材料進(jìn)行微觀形貌和電荷轉(zhuǎn)移水平的表征和分析，結(jié)果表明C60對(duì)sc-SWCNT 起到了p 型摻雜的作用。通過(guò)電學(xué)測(cè)試證明了器件對(duì)405～1 064 nm波段的可見(jiàn)-近紅外光具有較為靈敏的光電響應(yīng)，拓展了光電探測(cè)器在智能窗、人工智能眼鏡等下一代透明技術(shù)中的應(yīng)用。

2 實(shí)驗(yàn)

2.1 sc-SWCNT/ C60 全碳器件的制備

首先，制備(6,5)SWCNT 分散液。稱取0.5 mg(6,5) SWCNT 粉末（Sigma-Aldrich）分散到10 mL十二烷基硫酸鈉（SDS）水溶液（0.01 g/mL）中。冰浴超聲處理2 h 后，以14 000 rpm 離心30 min，去除分散不充分的 (6,5) SWCNT 粉末，將離心后的上清液稀釋5 倍，得到均勻的 (6,5) SWCNT 分散液。其次，通過(guò)真空抽取和過(guò)濾法制備GO 薄膜作為晶體管的介質(zhì)層，將GO 水溶液稀釋并低溫超聲處理1h 后，將10 mL 分散液逐漸加入含有0.22 μm 纖維素膜的真空過(guò)濾裝置中過(guò)濾，形成均勻的GO 薄膜，水溶液過(guò)濾后，加入過(guò)量的的去離子水清洗薄膜中多余的SDS 3 次，以減少其對(duì)薄膜性能的影響。最后，將纖維素膜上的GO 薄膜在40 °C 真空烘箱中干燥2 h。將氧化石墨烯水溶液替換為金屬性SWCNT 在去離子水中的均勻分散液（0.05 mg/mL）。用上述同樣的方法，真空抽濾得到均勻致密的導(dǎo)電sc-SWCNT 薄膜作為源漏電極的備用材料。

采用135 nm 厚的ITO 導(dǎo)電玻璃作為基底以及柵電極，首先，用去離子水、丙酮、異丙醇和乙醇依次清洗ITO 導(dǎo)電玻璃，去除基底表面油污。然后，將制備好的纖維素膜上的GO 薄膜轉(zhuǎn)移到ITO 基底上，利用乙醇和水將GO 薄膜貼合鋪展在導(dǎo)電玻璃表面，然后通過(guò)丙酮浸泡使纖維素膜溶解在丙酮中，并用丙酮反復(fù)清洗避免纖維素膜殘留在基底上影響器件性能。之后，通過(guò)旋涂法將上述制備好的sc-SWCNT 分散液以2 000 rpm的轉(zhuǎn)速沉積在GO 表面形成一層均勻的薄膜，并且通過(guò)真空熱蒸發(fā)法稱取2mg C60使之均勻蒸鍍到碳管薄膜表面，真空60 °C 退火處理1 小時(shí)，完成襯底上(6，5)SWCNT/ C60異質(zhì)結(jié)的構(gòu)筑。最后，將抽濾得到的金屬性SWCNT 薄膜通過(guò)重復(fù)上述步驟轉(zhuǎn)移到sc-SWCNT/C60異質(zhì)結(jié)上，形成源漏電極圖案，源漏電極之間的溝道寬度為200 μm，長(zhǎng)度為400 μm。

2.2 測(cè)試與表征

本文使用掃描電子顯微鏡（SEM, 德國(guó)蔡司Ultra Plus）對(duì)sc-SWCNT 薄膜/C60異質(zhì)結(jié)以及金屬性SWCNT 的表面形貌進(jìn)行了表征。用Lambda 950 型號(hào)的紫外-可見(jiàn)-近紅外分光光度計(jì)（美國(guó)）表征樣品對(duì)光的透過(guò)率，得到透射光譜。在激發(fā)波長(zhǎng)為514 nm 的條件下，通過(guò)拉曼光譜統(tǒng)計(jì)對(duì)異質(zhì)結(jié)的拉曼峰位移進(jìn)行分析。利用半導(dǎo)體參數(shù)分析儀在室溫下對(duì)器件的光電性能進(jìn)行評(píng)價(jià)，并在由功率可調(diào)的多種單色激光二極管作為光脈沖信號(hào)源（激光波長(zhǎng)包括405，532，650，780，860，940 和1 064 nm）的照射下測(cè)量了器件的電流-電壓（I-V）曲線和電流-時(shí)間（I-T）曲線。

3 結(jié)果與討論

構(gòu)筑的場(chǎng)效應(yīng)晶體管結(jié)構(gòu)圖如圖1(a)所示，其中ITO 和GO 構(gòu)成柵極和介質(zhì)層，sc-SWCNT/C60異質(zhì)結(jié)作為導(dǎo)電溝道，金屬性SWCNT 構(gòu)成源漏電極。如圖1(b)顯示了溝道（sc-SWCNT/C60薄膜）區(qū)域可見(jiàn)光范圍內(nèi)的光學(xué)透射光譜，其中插圖為制備好的場(chǎng)效應(yīng)晶體管的器件實(shí)物圖。可以明顯看出，ITO 上沉積的所有碳材料薄膜幾乎都是完全透明的，其透過(guò)率僅僅比襯底低10%，當(dāng)80%的襯底均覆蓋sc-SWCNT 薄膜（形成溝道和電極）時(shí)，該器件的光學(xué)透過(guò)率依然保持在80%以上，說(shuō)明器件的設(shè)計(jì)并不影響溝道層自身的光吸收。圖2(a)展示了通過(guò)旋涂法在GO 上沉積的碳管，sc-SWCNT 在整個(gè)區(qū)域均勻分布，并且在圖像中看不到明顯的表面分散劑以及其他聚合物。這證明納米管具有較高的分散性，超聲分散和離心的碳管溶液保證了sc-SWCNTs 薄膜的均勻性和致密性。圖2(b)是sc-SWCNT/C60復(fù)合薄膜的SEM圖像，從圖中可以看到，C60均勻分布在sc-SWCNT 薄膜中。圖2(c)展示了作為源漏電極的金屬性SWCNT 的SEM 圖像，可以看出，通過(guò)抽濾法形成了均勻致密的金屬性 SWCNT 薄膜，可以保證源漏電極具有良好的導(dǎo)電性。

Fig.1 (a) Schematic diagram of the device structure.(b) Transmittance of the device in visible band圖1 (a)器件結(jié)構(gòu)示意圖和(b)器件在可見(jiàn)光波段的透射率

Fig.2 Scanning electron microscope images of (a) sc-SWCNT film deposited by spin coating, (b) sc-SWCNT/C60 heterogeneous composite film and (c) m-SWCNT film圖2 (a)旋涂法沉積的sc-SWCNT 薄膜、(b) sc-SWCNT/C60 異質(zhì)結(jié)復(fù)合薄膜和(c)金屬性SWCNT 薄膜的掃描電子顯微鏡圖

拉曼光譜是研究碳納米管最常用的工具，可以使用拉曼光譜來(lái)分析其他材料對(duì)碳納米管的摻雜效應(yīng)[25]。通過(guò)觀察拉曼圖像中G 峰和2D 峰的變化，可以分析摻雜水平的變化。圖3(a)和3(b)為在G 和2D 模式下原始和C60摻雜的sc-SWCNT 薄膜的拉曼光譜。對(duì)于sc-SWCNT，1 300～1 400 cm-1區(qū)域的D 峰源于碳原子中的sp3缺陷[26]。在1 580 cm-1附近的強(qiáng)切向模（G 峰）則產(chǎn)生于sc-SWCNT 中sp2雜化C=C 鍵的面內(nèi)振動(dòng)。Sc-SWCNTs 的G 峰由G+（LO）和G-（TO）兩種模式組成[27]，所分析的LO 聲子代表的G+峰以及2D 峰的模擬峰在圖中顯示為淺灰色。在sc-SWCNTs中加入C60后，觀察到G+和2D 兩種主要峰的增寬和向能量更高的方向轉(zhuǎn)移的趨勢(shì)，這種變化可以歸因于C60對(duì)sc-SWCNT 的摻雜效應(yīng)?？梢杂^察到，加入C60后，sc-SWCNT 的G+峰半峰寬從原來(lái)的20 cm-1增寬到24 cm-1，此外，峰位置向右移動(dòng)了2.5 cm-1，2D 峰的拉曼光譜也產(chǎn)生了半峰寬增寬（約4cm-1）和輕微的藍(lán)移（從純sc-SWCNT 的2 634 cm-1到C60摻雜后的2 636 cm-1）。這些變化可能是由C60引起的p 型摻雜導(dǎo)致電子從sc-SWCNT 的價(jià)帶轉(zhuǎn)移到C60的跡象[28]。這表明，sc-SWCNT 與周圍C60之間存在電荷轉(zhuǎn)移，C60對(duì)(6,5) SWCNT 造成了p 型摻雜。

Fig.3 Raman statistical analysis of sc-SWCNT /C60 film.Raman spectra of (a) sc-SWCNT (black) and (b) sc-SWCNT /C60(blue) under 514 nm laser irradiation圖3 sc-SWCNT/C60 薄膜的拉曼統(tǒng)計(jì)分析。在514 nm 激光輻照下(a) sc-SWCNT (黑色)和(b) sc-SWCNT/C60 (藍(lán)色)的拉曼光譜

通過(guò)上述對(duì)sc-SWCNT /C60薄膜的拉曼分析，初步闡明了本文制備的場(chǎng)效應(yīng)晶體管的異質(zhì)結(jié)溝道的電荷轉(zhuǎn)移機(jī)制以及工作機(jī)理。首先，當(dāng)sc-SWCNTs 在光照下照射時(shí)會(huì)產(chǎn)生激子。由于C60的最低位占據(jù)分子軌道（LUMO）低于sc-SWCNT 的導(dǎo)帶，所以C60的光激發(fā)電子不能轉(zhuǎn)移到sc-SWCNT 的導(dǎo)帶[29]，又由于LUMOC60與sc-SWCNT 導(dǎo)帶之間的偏移能量差，大于產(chǎn)生的激子結(jié)合能，因此sc-SWCNT 價(jià)帶的電子可以轉(zhuǎn)移到C60最高占據(jù)分子軌道（HOMO）中[30]，產(chǎn)生電荷分離，即光生電子從sc-SWCNT 轉(zhuǎn)移到周圍的C60，sc-SWCNT 中空穴的密度同時(shí)增加。因此，由于所設(shè)計(jì)的異質(zhì)結(jié)結(jié)構(gòu)，sc-SWCNT 中的光生激子被有效地解離，降低了光生激子的重組率[31-32]。這種有效的激子收集使該器件表現(xiàn)出卓越的光電特性。

該異質(zhì)結(jié)器件的電學(xué)性能如圖4 所示。在源漏電壓（Vds）不變（Vds= 0.1 V）的情況下，通過(guò)改變柵壓（Vgs）調(diào)節(jié)溝道中的載流子濃度，從而改變?cè)绰╇娏鳎↖ds），得到轉(zhuǎn)移特性曲線，如圖4(a)所示。從圖中可以看出，特性曲線的轉(zhuǎn)折點(diǎn)約在Vgs= 1 V 左右，表明期間使用的溝道材料sc-SWCNTs 是p 型摻雜，當(dāng)施加的柵壓小于轉(zhuǎn)折點(diǎn)，即Vgs位于左側(cè)時(shí)，隨著柵壓絕對(duì)值的增大，溝道中感應(yīng)出越來(lái)越多的空穴。作為多數(shù)載流子，器件的源漏電流快速增大，相反，柵極電壓越接近轉(zhuǎn)折點(diǎn)，溝道中的空穴濃度就越低，源漏電流減少直至器件關(guān)斷，但是由于溝道材料是p 型摻雜的，器件不能完全關(guān)斷，仍然有約0.5 nA 的關(guān)電流。當(dāng)柵壓再次增大，高于轉(zhuǎn)折點(diǎn)時(shí)，溝道中感應(yīng)出電子，電子濃度隨著柵壓的進(jìn)一步增大而增大，逐漸取代空穴成為多數(shù)載流子，并使源漏電流反方向逐漸增大。通過(guò)計(jì)算得出該器件的跨導(dǎo)為2.268×10-8S，開(kāi)關(guān)比可達(dá)到500。在柵壓Vgs保持不變的條件下，源漏電壓Vds從-1 V 到 1 V 線性掃描得到器件的輸出特性曲線，測(cè)試一系列Vgs值下的輸出特性即可得到輸出特性曲線簇，如圖4(b)所示?？梢钥闯?，源漏電流在不同的柵壓下表現(xiàn)出明顯的分離，說(shuō)明該器件具有較強(qiáng)的柵控能力。同時(shí)，該器件由sc-SWCNT / C60復(fù)合薄膜在源極和漏極之間架起橋梁，全碳異質(zhì)結(jié)特殊的電學(xué)和光學(xué)性質(zhì)為高性能寬光譜的光電探測(cè)提供了材料基礎(chǔ)，因此本文在室溫空氣中研究了器件的光電性能。圖5(a)和5(b)分別展示了異質(zhì)結(jié)器件在不同波長(zhǎng)激光照射下的I-V 曲線以及在無(wú)偏壓下的I-T 曲線。為了測(cè)試器件的光電性能，分別在405、532、650、780、860、940、1 064 nm波長(zhǎng)的激光下（激光功率均為5 mW/cm2），使用半導(dǎo)體參數(shù)分析儀測(cè)試了偏置電壓Vds從-1 V 到1 V 線性掃描下的源漏電流，可以觀察到在1 V的偏壓下，可以產(chǎn)生約0.7 μA 的感應(yīng)光電流。當(dāng)激光反復(fù)打開(kāi)和關(guān)閉時(shí)，光電流分別相應(yīng)地急劇增加和減少，即該光電探測(cè)器在開(kāi)關(guān)狀態(tài)之間能夠表現(xiàn)出較好的重復(fù)切換性能，器件具有較為優(yōu)異的可靠性和循環(huán)穩(wěn)定性。另外，該器件在405～1 064 nm 內(nèi)不同波長(zhǎng)的激光下都有較為明顯的光響應(yīng)，證明了器件具有從可見(jiàn)光到近紅外的寬光譜光電響應(yīng)，并且在近紅外波段顯示出了一定的優(yōu)勢(shì)。為了進(jìn)一步評(píng)價(jià)該異質(zhì)結(jié)器件的光電性能，根據(jù)測(cè)試得到的光電流，分別計(jì)算了不同波長(zhǎng)激光照射下的響應(yīng)度和探測(cè)率。響應(yīng)度定義為每單位面積每單位照明激光功率密度下產(chǎn)生的光電流，計(jì)算公式為，其中Ip為感應(yīng)光電流，Id為無(wú)激光照射時(shí)的暗電流，P為光功率密度，A為器件溝道有效區(qū)域的面積。探測(cè)率一般作為探測(cè)器探測(cè)最小光信號(hào)能力的指標(biāo)，可以用來(lái)評(píng)價(jià)器件的靈敏度計(jì)算公式為，其中q為基本電荷，R為響應(yīng)度。計(jì)算可得，在功率密度為5 mW/cm2的940 nm 激光照射下，該器件可以達(dá)到18.65 A/W 的響應(yīng)度和5.35×1011Jones 的探測(cè)率，較好的光電響應(yīng)性能源于sc-SWCNT 超高的載流子遷移率、優(yōu)異的光吸收系數(shù)、較長(zhǎng)的激子擴(kuò)散長(zhǎng)度以及C60對(duì)激子有效解離和轉(zhuǎn)移收集。

Fig.4 (a) Ids-Vgs curve and (b) Ids-Vds curve of the all-carbon device圖4 全碳器件的(a)Ids-Vgs 曲線和(b)Ids-Vds 曲線

Fig.5 (a) Ids-Vds curve and (b) Ids-T curve of the all-carbon device under 405, 514, 650, 780, 860, 940, 1 064 nm laser irradiation圖5 全碳異質(zhì)結(jié)器件在不同波長(zhǎng)（405, 514, 650, 780, 860, 940, 1 064 nm）激光照射下的(a) Ids-Vds 和(b) Ids-T 曲線

4 結(jié) 論

利用sc-SWCNT 獨(dú)特的電子特性和高載流子遷移率，本文設(shè)計(jì)并制備了一種透明、全碳場(chǎng)效應(yīng)晶體管型光電探測(cè)器。該場(chǎng)效應(yīng)晶體管的組件（有源通道、源漏電極、介電層）都由碳基材料組成，可見(jiàn)光波段透光率均高于80%。除了sc-SWCNTs 提供的大吸收系數(shù)和高傳導(dǎo)路徑外，sc-SWCNT 與C60的界面之間有適當(dāng)?shù)哪軒帕校馍娮?空穴對(duì)的分離也使器件的光電響應(yīng)性能得到了顯著提升。通過(guò)對(duì)器件進(jìn)行電學(xué)測(cè)試，證明了器件具有較好的柵壓控制能力，940 nm 下的響應(yīng)度和探測(cè)率分別可達(dá)18.65 A/W和5.35×1011Jones，并且可以實(shí)現(xiàn)405～1 064 nm 寬光譜響應(yīng)，證明了sc-SWCNT/C60薄膜異質(zhì)結(jié)用于光電探測(cè)器的巨大優(yōu)勢(shì)。本文所展示的結(jié)果為高性能全碳基光電器件的設(shè)計(jì)與制備提供了一個(gè)新的思路，為下一代全碳透明光電子技術(shù)的發(fā)展提供了有力的支撐。